Differential refractometer



March 3, 153

H. SHEFFER ET AL 2,630,042

2 SHEETSSHEET 1 VII/Ill/IIl/IIII 17 I5 /NVENTOR$ HENRY JOHN HADOW HARR Y SHE/FER J HN COL/N HYDE ATTQRNEYS Patented Mar. 3, 1953 UNITED STATES Q CE 1 This invention relates to 'a refraotometer and in particular to a differential .refractometer for finding the difference in the refractive indices of two liquids which obtains a greater measure of accuracy than existing devices by supplying a larger number-ofretracting interfaces.

Difierential refractometers are known which have in planview arectangularsolvent cell and a triangular solution cell contained therein. The triangle has its base on the long side of the rectangle and has roughly the same height as the Width of the rectangle. The ray of :light entering the cell parallel to thelongside of the rectangle and the base-ofthe'triangle, is refracted at two solutionesolvent interfaces. *Inthis way the slight bending of the teeam due to the difference in refractive indices takes place twice and the measurable deflection increased. An instrument designedaccording to this principle is sufficiently accurate for most uses. However for certain solutionsthe difference in refractive indices'between solution and solvent is sufficiently small that the above describedinstrumenthas not the sensitivity to give accurate results.

It is the object of this invention to providea differential refractometer having greatersensi tivity, than hitherto.

It is the further object of this-invention to provide a diiferential refractometer having greater sensitivity, than hitherto by provision of acompact liquid retracting area. anda compact device which embodiesthe optical lever principle.

The device embodying this invention provides four interfaceswhich. are-cutob1iquely by alight ray and the light ray thereby bent at ,eachof the four interfaces due to the difference -in-refractive indicescn each sideof the interface, the device being-so constructed that this bending is additive, so that a :greater angular deflection and consequent greater, sensitivity is obtained with this device than With'instruments upqto this time.

The invention does not however lieialone in the provision of four interfaces-since this-would be easily attainable by -severaltypes of sufliciently elaborate apparatus, but lies in providingasimple construction wh ch uses a minimumfof the scarcer liquid. Furthermorethe.deviceis so designed that the hending-which takes place atthe interfaces is. additive.

If a light ray strikes .theinterface'between two materials of different refractive indices, bending takes place unless the direction of the ray is normal to the interface. If the light raypasses from one liquid to anotherwith the interface sloped in one direction relative to the normal it is bent in a certain sense. of rotation. If the .Iray then goes from the second liquid to the first through a second interface, slopedinthe opposite direction relative to the normal, pending again k s p aceWhishdain ;ihasamassaeap 1 t e TQQmQ i a the longdiagcnalof .the'..rhombiis and on being passe similarly t thetotal refraction.

. .theless available l'diaicl.

rotation as the first bending, This much is achieved by the priorart devices disclosed above.

The ray having thus been twice refracted, .leavesthe liquid region, is reversed and displaced, re;enters the liquid region and is twice more refracted at the other two interfaces, the placing of the liguids .and the slopes of these interfaces being such that the second series of bendings is additive to the first. The idea of reversing and displacing the light ray half way through the bending operation allows the apparatus, which permits this additive-bending, to "be made compac sim y- By providing an outer cell having the faces,

.throughwhich the ray ipass es normalto the ray, and placing therein .an inner cell having four -.faces which are intersectedlby the ray at angles alternatelyon' qppcsite s ides ofthe normal, it is seen that if twoliquidsftthe difference of Whose refractive indexitisdesiredto find) are placed respectively in the finner land outer cells, the ray will .pass alternatel H oni'me'diums of higher'to iovr and .froin m unis of lower tohigher refract ve index; .Cifhus theibendin'g of the ray at the interfaces will beadditive.

The embodimentofthe invention specifically dcscrib ed be Iut es ,la',n inner cell having the shape of a rh stantially parallel to the longsideofare ng laroutercell. Alight ray from source .passes'Ithroughtwo interfaces which in. planldefin one ofthecbtuse angles of ction roughly parallel to sed. displaced byfbeing gdoubl'y reflected, gh' the other two interfaces, to ell for nieasurement of ,leaveslthe c m Although therhoinbus shape-is shown specifioaliy andwaschcsen forfacility of manufacture and ease of construction, itsvillbe obvious that several other-shapes wiil ive the same effect and. utilize the e entialfeaturesof this invention,

since the invention is directed to a refractoineter so constructed that thelight from the source intersects 1am oppositely sloped faces in passing through thecom'pcsite qell is thenreversed and displaced, rby doublelreflecltion, and intersects two more oppositelysloped facesin returning through the composite" cell.

It willbe seen that the solution could be placed or center cell and the solvent houttleparting from the essence of the inven ni although thesense ofadditive bending .willb appropriate measure ing. The .rhjo'mbus'i 'vje sod having regard to tpfth additive bendade small injrelat on to the outer .cell tolreduc ,the quantity required of Another,invent vej feature is disclosed .Wherein the refractive effects are ma nifiedby combining th l te ch s ahevawih 9 ta device to the sensitivity of the instrument as a whole.

This latter device obtains the advantages accruing from the use of an optical lever without necessitating the space usually required by such devices by providing two parallel mirrors between which the ray is several times reflected and thus travels the long optical path length between the two mirrors so that the instrument as a whole may be made very compactly.

The invention will now be described in detail with reference to the attached drawings in which:

Figure 1 shows a plan view of the invention and the apparatus used therewith.

Figure 2 shows a plan view of the reiractometer cell and the optical path therethrough.

Figure 3 shows an alternate form of the re fractometer cell in order to show the scope which the invention is intended to include.

Figure 4 shows the optical lever device suggested for use with the refractometer in plan View.

Figure 5 is an elevation of the optical lever device shown in Figure 4.

Figure 6 shows a calibration graph showing the linear relation between the difference in refractive index and the ray deflection.

The source it may be any lamp which emits a widely spaced spectrum of strong wavelengths. The reason that the wide spacing is desirable is so that the monochromatic light may be obtained by the use of filters. The source may be, for ex ample a mercury or sodium lamp. Light from the source It is passed through a small adjustable slit i which allows diverging light to fall on a lens l2 which is so placed so that the emergent rays are parallel. In line with source, slit and lens is a filter or combination of filters is which removes the unwanted wave lengths from the light beam thus supplying substantially monochromatic light. (In the case of a mercury lamp in order to use the 54:61 A. line, a filter such as that known by trade name Corning No. 3484 is used to eliminate all wave lengths below 5200 A. and in combination with this, a filter known by the trade name Corning No. 5120 is used to eliminate the strong band at 5770 A. in order to obtain the 4358 A. line, filters known by the trade name of Corning Nos. 5113 and 3389 are used.)

An outer cell 54 of rectangular shape encloses an inner cell i5 therein, both cells having horizontal top and bottom walls which may be common to both cells, the long sides of the outer cell being parallel to the optical path delineated by the arc, slit, lens and filter before described, but the center line of the inner cell is below the optical path delineated (using the orientation of Figure 1). The inner cell it has the shape of a rhombus withits long diagonal parallel to the aforementioned optical path and is centrally disposed in the outer cell. The outer cell is constructed of glass while the inner cell is constructed of metal e. g. stainless teel having holes to allow passage of the light along the paths described herein and having glass windows cemented over the holes.

The stainless steel construction of the inner cell provides a separating surface of high heat conductivity whereby the liquids in the inner and 7 outer cells tend to rapidly reach temperature equilibrium, temperature equilibrium being very important in the measurement of refractive index difference.

If aqueous solutions are to be used in the measurements of refraction there are many cements which may be used, but for organic solvents many of these cements are unsatisfactory and it is suggested that a cement made of gum arabic, sucrose and water be used.

The rhombus is herein disclosed as having obtuse corners of and acute corners of 50 but it will be seen that many other angles could be used and still fall within the ambit of this invention.

A 45 prism 16 has its large face parallel to the end wall of the rectangle delineated by the outer cell in Figure 1 and with the right angled edge of the prism l6 lying in the same Vertical lane as the center line of the outer cell, said right angled edge being on the side of the large face farthest from the outer cell.

The essential elements of the invention so far described are the composite cell composed of outer cell i and inner cell it: and prisms it and ii. These components are mounted in a double walled closed container it, through which a thermostatically controlled liquid is circulated to maintain a fixed uniform temperature on all sides. The container also has insulating material applied to all outside walls.

The composite cellis mounted on a holder in the container in such a, manner that the cell may be moved completely out of the path of the beam for the purposes of obtaining a zero correction without opening the thermostated container. Glass covered apertures l9 and 2i] allow the beam to enter and leave cell container l8.

it is seen that light from the source id falls upon the slit l i and forms a source through which diverging emerges, this light falling on the lens i2, the slit i! being at the focal length of the lens i2 50 that light from the slit emerges in a parallel direction from the lens, becomes monochromatic by passing through the filter 23, enters the upper portion of the solvent cell (using the orientation of Figure l) and strikes the solvent-solution interface I1 and the refractive index of the solution being greater than that of the solvent the light ray is bent clockwise (as seen in Figures 1 and 2). Emerging from the solutionsolvent interface I2 the difference of indices again causes the beam to be bent clockwise and it emerges from the solvent cell having been twice defiected. The beam then strikes the reflecting prism and is reflected roughly at right angles by the face P1 of the prism i8, is then reflected in a direction substantially opposite to original direction by the face P2 in the prism, then bent clockwise twice in passing through the interfaces Is and I4 and then strikes the face P3 of the prism i1 and is reflected out of the holder 58. It is thus seen that the beam is deflected four times along this path (due to the refractive index difference between solution and solvent) and the angular deflection therefore is substantially greater than that which can be expected with of the instruments now in use. The deflection of the beam may be measured by a travelling microscope placed in the path of the beam as it emerges from the prism ll and from the holder.

It will be seen that the ray on emerging from the outer cell after the deflections before described might be reflected out of the plane of its path (1. e. the plane of the paper in Figure l) and into a parallel plane which also cut the cells as seen in Figures 1 and 2. The ray could then be made to follow a similar path to that shown in those figures so that it is subjected to a total of eight deflections instead of four, and after these deflections the total deflection could then be measured. The total deflection is increased (substantially doubled) by this second travel around the cell and if desired the cell and reflecting means may be so designed that as many circuits may be made, the limiting factor being the number of circuits at which th absorption of the ray is so great as to prevent the accurate observation and deflection measurement of the ray when it finally emerges from the cell.

It is now proposed to describe a suggested optical lever method by which the deflection obtained by using this invention may be increased and the sensitivity of measurement measured by travelling microscope proportionally increased. This is shown in Figures 4 and 5 wherein the beam emerging from the holder and prism i: is shown as coming from the point A. This beam strikes a lens 2| of long focal length (such as a 3 meter lens) and thereafter impinges upon the short face of a 45 prism 22 which is rotated slightly so as to cause the previously horizontal beam to be deflected downwards at a slight angle to the horizontal. This beam is reflected and forth between plane parallel mirrors 23 and 24 until it reaches deflecting mirror 25 situated below mirror 24, from which it is deflected to a travelling microscope 28 which views an image of slit il situated at an optical path length equal to the focal length of lens 2 I.

The horizontal displacement of the beam at this point is a magnification oi the shifting of the beam due to the accumulated bending at the interfaces caused by the refractive index differences of the two solutions.

The vertical displacement of the beam is controlled by the amount of rotation of prism 22. This is determined by the distance between the parallel mirrors and the number of reflections necessary to accommodate the focal length of lens 2 i. It is seen that the deflection of the beam is increased by this principle. The use of the above described optical lever device in combination with the refraction cell has reduced the maximum dimension of the instrument herein described to about one quarter of that of existing difierential refractometers employing an optical lever principle. Although this method is suggested for compactness, any optical lever principle may be used.

Having described the invention in detail it is proposed to note the techniques which were found advisable in the use of this apparatus.

The apparatus is calibrated by a convenient refractometer such as that known as the Pulfrich refractometer. In the calibration a 4% solution of cyclohexane in benzene was prepared and the desired refractive index increment obtained. The deflections for several dilutions of this solution were measured on the differential refractometer. Figure 6 shows the sensitivity and linearity of the apparatus: the ordinate showing distance on the microscope scale and the abcissa showing the diiierence in the refractive indices of two liquids. In obtaining the zero reading for the differential refractometer it is necessary to correct for a small shift produced when both cells are filled with solvent. This occurs if the faces of the rectangular cell are not exactly parallel to one another and it is of course very difilcult to obtain this condition to a high degree of precision. The zero of this device should be checked after each reading since it has been found that in spite of the temperature control, changes in room temperature cause a shift in the zero reading. It has been found with the apparatus disclcsed specifically herein, that two hours are required to attain a true temperature equilibrium.

In making measurements with this device it should be noted that the same sample of solvent must be used in making up a solution and in the measurements since appreciable differences in refractive index due to of impurities such as water were found with samples of reagent benzene from different bottles.

We claim:

A differential refractometer for measuring the differences between the refractive indices of two liquids comprising a substantially rectangular outer cell containing one of said liquids having side walls end walls, an inner cell containing the other of said liquids disposed within the outer cell and having four sides, each of said sides being disposed facing a corner of the outer cell, means for transmitting a beam of light successively through one end of the outer cell, obliquely through the two sides of the inner cell which are adjacent to one side wall with the outer cell and through the other end wall of the outer cell, said means for transmitting a beam of light comprising a light source and collimating slit, a lens between the light source and the cells for condensing light from said light source into a substantially parallel beam; means for reversing and displacing the beam and directing it successively through the last mentioned end wall of the outer cell, obliquely through the two sides of the inner cell which are adjacent to the outer side wall of the outer cell and through the first mentioned end Wall, and means for measuring the displacement of the beam due to the cumulative refractions of the interfaces between the liquids in the inner and outer cells, said measuring means comprising a lens for converging the beam, an opti-- cal lever including a pair of opposed parallel mirrors and a travelling microscope for viewing the image of the slit and measuring its deflection, said parallel mirrors and said microscope being disposed so that the optical path length between the lens for converging the beam and the microscope is substantially equal to the focal length of the last mentioned lens.

HARRY SHEFFER.

JOHN COLIN HYDE.

HENRY JOHN HADOW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 454,989 Sonden June 20, 1891 740,484 Stocoum Oct. 6, 1903 1,264,374 De Florez Apr. 30, 1918 1,727,173 Muller Sept. 3, 1929 1,736,682 'I uckerman Nov. 19, 1929 1,770,355 Doi July 8, 1930 1,923,891 Skaupy Aug. 22, 1933 2,427,996 Seaman Sept. 23, 1947 2,445,044 Stamm et a1 July 13, 1948 FOREIGN PATENTS Number Country Date 22,594 Great Britain of 1913 OTHER REFERENCES Baly, E. C. C. text; Spectroscopyvol. 1, pages 51-53, 3rd edition, 1924-publ. Longmans, Green & Co., New York city. (Copy in Division 7.) 

